The present invention is related to thermocompression bonding of wire using opposing electrodes, and is more particularly related to testing such thermal compression bonding.
Prior techniques for attaching wire to straddle mount connectors required the use of a printed circuit card which was used as the interface to the connector. The wires were attached via solder to reflow to the card. On larger gauge wires (26 AWG+), a new technique had to be developed since printed circuit cards would not withstand the high temperatures required to attach the wires without causing delaminations. Resistance welding was considered for directly attaching wires to a connector thereby eliminating the printed circuit card. However, resistance welding requires heating the wire and connector to their melting temperatures, and the melting of all metals in the weld nugget, including coatings and the base metal. In resistance welding, material selection and the thickness of the material is critical. Resistance welding produces a weld which results in a fusion/melting of materials which in turn create alloys of different material characteristics than the original metals such as brittleness due to alloying effects.
In the present invention, a thermocompression bond is made between a wire and a connection by use of opposing electrodes. The thermocompression bond is made while heating the materials to temperatures below their melting temperatures, and the resulting bond is made by diffusion rather than fusion. The geometry is easy to maintain, and most materials can be adapted. The present invention includes testing the resulting bonds to insure that the bonding setup for the materials is correct. In the invention, a sample of the bonds are subjected to a peel test which includes bending the bonded wire at a right angle to the connector, securing the connector, and applying pulling force at a constant rate of 50 mm (about 2 inches per minute) to the wire until the wire is separated or peeled from the connector. Indications of the bond is defined by wire peel force being greater than 50% of the wire tensile strength with a minimum Cpk (capacity index) of 1.5, and with wire deformation being less than 50% of the outside diameter (OD) of the conductor. In addition to the peel test, a pull test may be conducted. The pull test involves an axial pull of the wire at a constant rate of 50 mm (about 2 inches per minute). The pull test shall not show less that 50% of the wire tensile strength with a Cpk of 1.5. The data must be normally distributed to calculate a valid Cpk. The formula for Cpk is as follows:
Cpk=(Xdouble barxe2x88x92LSL)/3"sgr"
Where:
Xdouble bar=the average of all subgroup averages or the grand average,
LSL=the lower specification limit, and
"sgr"=the standard deviation.
Using well known statistical process control on the peel force allows for the desired type of bonding to be continuously achieved.